The humoral and lymphocyte membrane receptor function of IgD will be investigated using sensitive radioimmunoassay and single cell techniques. In addition the ontogeny of IgD and of IgD-producing cells will be studied in the patas monkey. Patas monkeys will be immunized with a variety of antigens chosen on the basis of human responsiveness. These antigens will include: a potent stimulator of IgE antibody since IgD alterations have been demonstrated in human allergy, an antigen which induces experimental autoimmunity since IgD autoimmune antibodies have been demonstrated in humans, Group A streptococcal vaccine since IgD has been observed to increase in chronic infections. Oral immunization will be used in some animals since IgD antibodies have been identified in intestinal secretions. Serum from immunized animals will be examined for IgD antibodies by radioimmunoassay. Cells from peripheral blood and tissues will be examined for membrane and cytoplasmic IgD fluorescent antibody assay. Antigen-binding cells will be detected by autoradiography and the class of immunoglobulin associated with antigen-binding will be identified by fluorescent antibody and capping techniques. IgD receptor function will also be studied by examining the effect of anti-IgD on mitogen-induced lymphocyte blastogenesis and on in vitro primary and secondary plaqueforming cell responses. This study will establish an experimental animal model for the study of IgD and give information concerning the role of IgD as a humoral antibody and as a lymphocyte membrane receptor.